Clarifiers of various sizes and types are commonly used in applications requiring the settling of solids from an influent liquid, such as in the treatment of water and wastewater. The influent is usually introduced into the clarifier from an influent column which extends in the center of the clarifier and has ports that discharge the influent from the column. Clarifiers are normally equipped with feedwells that take the form of rings extending around the influent column near the top of the clarifier basin.
There are a number of flow characteristics of the influent that have been recognized as being problematic. First, the influent discharges from the center column at relatively high energy levels that must be dissipated to reduce the influent velocity so that the flow into the clarification zone is uniform. In the past, inlets such as shown in U.S. Pat. No. 2,635,757 have been used to direct the flow tangentially into the feedwell and also with a radially outward component. In time, it was discovered that non-uniform radial flow was undesirable because it tends to maintain a circulation pattern that is detrimental to settling.
More recently, other types of inlet structures have been proposed. One that has been largely successful makes use of an inlet structure that extends around the influent column and is constructed to achieve tangential flow into the feedwell through outlets equipped with scoops. Another inlet structure has used box-type baffles to discharge the influent horizontally in each direction. U.S. Pat. No. 6,276,537 provides a bottom discharge into tees that direct the flow in opposite directions. Recent patent publication No. 2004-002847-A1 is directed to an arrangement that intermixes the influent using directional baffles.
While all of these approaches to dissipating the inlet energy are improvements over simple ports in the influent column, they all have drawbacks. One common problem is that the influent enters the feedwell in a number of separate or discrete flow streams that are at relatively high velocities and energy levels. These separate flow streams cause turbulence, and they tend to maintain their separate identities in the clarifier zone, thereby reducing the efficiency and the clarifier capacity. Another problem is that when the influent column ports are aligned with the scoops of the inlet structure, imbalances in the flow occur and relatively high velocities result at the locations of the alignments. Such flow imbalances and non-uniformities in the flow detract from the performance of the clarifier to a significant extent.
The depth of the feedwell is known to have a significant effect on clarifier performance. The liquid must pass beneath the lower edge of the feedwell in order to enter the clarification zone. The deeper the feedwell is, the less distance there is between its lower edge and the basin floor. A small clearance between the feedwell and clarifier floor is undesirable because it results in a higher flow velocity which reduces the hydraulic detention. The built up sludge on the floor can be disrupted by high velocity flow into the clarifier from the feedwell. Thus, a shallow feedwell is recognized as being advantageous to clarifier performance.
As a practical matter, the feedwell depth should ideally be no more than about 45% of the depth of the clarifier wall in order to achieve good hydraulic detention and avoid scouring of the sludge blanket on the clarifier floor. However, there are many existing basins that are shallow and others that must be shallow because of site conditions or other factors. Therefore, it is often not possible under currently prevailing design practices to reduce the feedwell depth to less than 50% of the sidewall depth.
The main reason is that the feedwell must extend below the bottom of the discharge from the inlet structure far enough to provide sufficient clearance that the influent flow cannot short circuit directly under the feedwell. The clearance between the bottom of the discharge from the inlet structure and the bottom of the feedwell should be at least three feet if short circuiting of the influent is to be avoided. In order to produce acceptable flow velocities into the feedwell, the ports must have a relatively long height dimension so that their combined area is large and yet they are still far enough apart that the center column structure has sufficient strength to withstand the forces to which it is subjected. As a result, the discharge outlets from the inlet structure are typically about two to three feet deep at their lower edges, resulting in the feedwell being about five to six feet deep at its lower edge. In shallow clarifiers, the clearance between the feedwell and the clarifier floor is insufficient to achieve good clarifier performance under these conditions.